ECOLOGICAL CONSEQUENCES OF THE ONSET OF MEROMIXIS DURING RESTORATION OF MONO LAKE, CALIFORNIA, USA

Hypersaline Mono Lake constitutes a productive ecosystem. High rates of pelagic primary productivity (ca. 500 g C m-2) support a large brine shrimp population that is a major food source for a significant portion of North America's breeding California Gulls (>20%) and migrating Eared Grebes (ca. 75%). Diversions of freshwater streams out of the Mono Lake Basin since 1941 led to a gradual decrease in size, an approximate doubling of lake-water salinity, and associated environmental impacts. To partially restore pre-diversion conditions, the State of California decided to restrict diversions until the lake rises 6 m above its 1982 low stand. Curtailment of diversions and above average runoff in the mid-1990s caused a large rise in surface elevation and the onset of persistent chemical stratification (meromixis). The episode of meromixis is expected to last several decades given the current management policy. The absence of a winter period of holomixis in which nutrients are recycled to the euphotic zone has caused declines in primary and secondary productivity and altered the temporal pattern of brine shrimp (Artemia) abundance. This has contributed to low productivity of the gulls and reduced body weight of grebes.